U.S. patent number 6,944,221 [Application Number 08/901,338] was granted by the patent office on 2005-09-13 for buffer management in variable bit-rate compression systems.
This patent grant is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Gerrit J. Keesman.
United States Patent |
6,944,221 |
Keesman |
September 13, 2005 |
Buffer management in variable bit-rate compression systems
Abstract
A method of compression is provided for transmission of digital
video signals between an encoder buffer and a decoder buffer. A
tunable delay is provided at the encoder, suitably in the form of a
portion of encoder buffer memory, with data being read out to a
communications channel at a rate determined by the input bit rate
of signals received a predetermined number of frame periods later.
Use of the tunable delay improves efficiency of decoder buffering
by maintaining a relatively constant level of decoder buffer
fullness irrespective of signal bit rates.
Inventors: |
Keesman; Gerrit J. (Eindhoven,
NL) |
Assignee: |
Koninklijke Philips Electronics
N.V. (Eindhoven, NL)
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Family
ID: |
23442599 |
Appl.
No.: |
08/901,338 |
Filed: |
July 28, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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366339 |
Dec 28, 1994 |
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Current U.S.
Class: |
375/240.02;
375/240.03; 375/240.04; 375/E7.216; 375/E7.244; 375/E7.014;
375/E7.184; 375/E7.159; 375/E7.179; 375/E7.162; 375/E7.134;
375/E7.139 |
Current CPC
Class: |
H04N
19/124 (20141101); H04N 19/184 (20141101); H04N
19/14 (20141101); H04N 19/152 (20141101); H04N
21/44004 (20130101); H04N 19/61 (20141101); H04N
21/23406 (20130101); H04N 19/177 (20141101); H04N
19/50 (20141101); H04N 19/115 (20141101); H04N
19/146 (20141101) |
Current International
Class: |
H04N
7/50 (20060101); H04N 7/32 (20060101); H04N
7/24 (20060101); H04N 007/18 () |
Field of
Search: |
;348/415,416,419,420,390,398,387,461,463,432,431,384,405,409,423
;375/240.01-240.29 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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A2-0660612 |
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Jun 1995 |
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EP |
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A2-0664651 |
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Jul 1995 |
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EP |
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Other References
"Constraints on Variable Bit-Rate Video for ATM Networks", Amy R.
Reibman and Barry G. Haskell, IEEE Transactions on Circuits and
Systems for Video Technology, vol. 2, No. 4, Dec. 1992, pp.
361-372..
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Primary Examiner: Rao; Andy
Parent Case Text
This is a continuation of application Ser. No. 08/366,339, filed
Dec. 28, 1994 now abandoned.
Claims
What is claimed is:
1. A method of compression for transmission of encoded digital
video signals having a variable number of data bits per image
frame, comprising the steps of: a) detecting a first bit rate of an
encoded digital video signal bit stream; b) sequentially writing
the encoded digital video signal bit stream in an encoder buffer at
the first bit rate; c) deriving a second bit rate as a percentage
of the first bit rate, which percentage changes inversely in
relation to changes in the first bit rate; and d) reading the
encoded digital video bit stream out of the encoder buffer at the
second bit rate; and transmitting the encoded digital video bit
stream to a decoder buffer at the second bit rate.
2. A method as claimed in claim 1, wherein for a specified range of
first bit rates, the second bit rate equals the minimum first bit
rate in said range.
3. A method as claimed in claim 1, wherein successive groupings of
one or more image frames are specified, and the second bit rate is
derived from a bit rate of the first frame of the grouping and
maintained constant until the bit rate of a first frame of a
succeeding grouping is detected.
4. A method as claimed in claim 3, wherein for signals encoded
according to the MPEG standard, a first grouping is assigned to
I-pictures and a second grouping is assigned to other types of
images.
5. A video signal encoding apparatus for encoding a received
digital video signal for transmission, the apparatus comprising: an
encoder stage for encoding a received video signal according to a
predetermined coding algorithm and producing the encoded video
signal as a variable bit-rate data stream at an output of said
encoder stage; a buffer coupled to receive said variable bit-rate
data stream from the encoder stage and arranged to output a data
signal corresponding thereto for transmission; and means coupled to
said encoder stage to (i) detect the bit-rate of said variable
bit-rate data stream, (ii) derive a second bit rate as a percentage
of the detected bit-rate, which percentage changes in inverse
relation to changes in the detected bit rate, (iii) control said
buffer to produce said output data signal at second bit rate, and
(iv) transmit the output data signal to a decoder buffer at the
second bit rate; wherein the detected bit rate and said second bit
rate are variable.
6. Apparatus according to claim 5, wherein the encoder stage is
configured to encode the received video signal in accordance with
the MPEG standard.
7. Apparatus according to claim 5, wherein said means for detecting
a bit rate stores a plurality of contiguous ranges of bit rate
values and, on first detecting an encoder output bit rate falling
within a first of said ranges, maintains the derived second bit
rate substantially constant until a detected encoder output bit
rate falls with another of said ranges.
8. A method as claimed in claim 1, wherein an instantaneous bit
rate of an image frame of the encoded digital video signal bit
stream is inversely related to a bit density of an image frame of
said bit stream n frame periods later, where n is determined by
said bit density.
9. A method as claimed in claim 2, wherein successive groupings of
one or more image frames are specified, and the second bit rate is
derived from a bit rate of the first frame of the grouping and
maintained constant until the bit rate of a first frame of a
succeeding grouping is detected.
10. The method as claimed in claim 1, wherein the step of deriving
the second bit rate is carried out by changing the percentage of
the first bit rate in response to changes in the first bit rate in
such a manner as to maintain a substantially constant fullness
level of the buffer.
11. The apparatus as claimed in claim 5, wherein the means coupled
to said encoder stage derives the second bit rate by changing the
percentage of the first bit rate in response to changes in the
first bit rate in such a manner as to maintain a substantially
constant fullness level of said buffer.
12. A method of compression for transmission of an encoded digital
bit stream having a variable bit rate, comprising the steps of:
detecting a current bit rate of the encoded digital bit stream;
sequentially writing the encoded digital bit stream into a buffer
at the detected current bit rate; reading the encoded digital bit
stream out of the buffer at a buffer read bit rate; and, varying
the buffer read bit rate in such a manner as to maintain a
substantially constant fullness level of the buffer in response to
changes in the detected current bit rate, wherein the buffer read
bit rate is a percentage of the detected current bit rate, which
percentage varies inversely in relation to changes in the detected
current bit rate.
13. The method as set forth in claim 12, wherein a delay between
the input and output of the buffer varies as a function of the
detected current bit rate, so that the delay is relatively higher
for a detected current bit rate which is higher than a prescribed
bit rate and is relatively lower for a detected current bit rate
which is lower than the prescribed bit rate.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for the
compression of digital video signals and in particular to the use
of such method and apparatus in the encoding and decoding of
signals.
Variable bit rate (VBR) video compression is known to give
advantages over constant bit-rate (CBR) video compression. The main
reason for this is that in a CBR system the bit rate has to be set
so that the worst case quality is acceptable while in a VBR system
the bit rate is set so that the average quality (which is kept
constant) is acceptable. The difference between the average bit
rate of a VBR compressed signal compared to the bit rate of a CBR
compressed signal has been found to be to be close to 30% in favour
of VBR compression.
Though VBR compression is better than CBR, it can only be used in a
limited number of applications. In principle the medium should be
able to convey variable bit-rate signals. In terms of the ISO/OSI
model, almost every medium conveys a fixed bit rate at the physical
layer. On a higher level, the medium can be converted into a
(logically) variable bit rate medium.
One particular application of VBR compression is joint bit-rate
control, in which a number of sources make use of a single channel.
In the case of joint bit-rate control the bit-rate of the video
signals is controlled such that the individual bit rates can be
varying (through optimal allocation of bit rate) but so that the
sum of all bit rates is constant. This type of system may occur in
for instance cable television or in satellite television
services.
The idea of joint bit-rate control for multi-program video signal
encoding has been found to be advantageous, particularly for video
signals coded according to the ISO MPEG standards. Basically a
system for joint bit-rate control needs technical measures for two
problems, namely the bit allocation and the buffer management. To
deal with bit allocation, the bit need of the programs is measured
and the bits are spread accordingly over the programs. Buffer
management encompasses several sub-problems, some of which are
described in "Constraints on variable bit-rate video for
ATM-networks" by Amy Reibman and Barry Haskell; in IEEE
Transactions on Circuits and Systems for Video Technology, Vol 2,
No. 4 December 1992 pp. 361-372. The Reibman and Haskell paper
examines the constraints resulting from encoder and decoder
buffering in an asynchronous transfer mode (ATM) network, in
particular the additional constraints needed to prevent overflowing
or underflowing of the decoder buffer when a variable bit-rate data
channel links encoder and decoder. They describe a method in which
the number of encoded bits for each video frame and the number of
bits transmitted across the variable bit-rate channel are selected
jointly, a necessity imposed by the differing constraints imposed
on the transmitted bit rate by the encoder and decoder buffers
respectively.
In all these applications the transmission of video can be in the
MPEG format. An MPEG decoder contains a physical buffer, and
correct MPEG bit streams must fulfil the video buffering verifier
(VBV) constraints, which means that the signals may not overflow or
underflow a hypothetical decoder buffer. As will be shown
hereinafter, an incorrect buffer management may limit the
performance of a VBR compression system. The analyses of buffering
systems rely strongly on the concept of a system delay whereby, in
order to have a continuous display of video, the system delay must
be constant. This requirement has an effect on the buffering
strategy.
For CBR systems, given a fixed decoder buffer size, the delay must
be relatively large for a low bit-rate and relatively low for a
high bit-rate in order to make full use of the available buffering
space in the decoder. If the bit rate is variable a compromise
setting of the buffering system must be used. As a consequence of
this compromise the effective buffer size in the decoder is too low
for an acceptable performance at low bit rates. Essentially, two
things are required, namely a constant end-to-end delay, and a low
buffering delay for high bit rates and a high buffering delay for
low bit rates. These requirements would appear to be
incompatible.
It is an object of the present invention to provide improved
stability of decoder buffer operation.
It is a further object of the present invention to provide greater
efficiency in encoder buffer management.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a method
of compression for transmission of encoded digital video signals
having a variable number of data bits per image frame, comprising
the steps of: a) detecting a first bit rate of the encoded digital
video signal bit stream; b) sequentially writing the signal bit
stream into a buffer at the first bit rate; c) deriving a second
bit rate as a percentage of the first bit rate, changes are which
percentage are inversely relation to changes in the first bit rate;
d) reading the bit stream out of the buffer at the second bit
rate.
By the provision of the "tunable delay" resulting from the varied
output and input bit rates in the encoder buffer, the contents of
the nominal decoder buffer will remain substantially constant. The
relationship between input and output rates means that at high
input signal bit rates, where the remainder of the buffering system
(provided by the encoder stage and the decoder) provides a
relatively low delay, the tunable delay provides a relatively high
delay. At low input signal bit rates, the tunable delay is reduced
to counter the relatively higher delay introduced by the remainder
of the buffering system.
The range of possible input bit rates may suitably be divided into
discrete levels with a derived second bit rate being maintained
constant until an input signal bit rate within a different one of
the levels is detected at which point the second bit rate is
recalculated. For a minimum input bit rate, the second bit rate is
suitably set to equal that bit rate: in other words, at minimum
input bit rate, the delay is set to zero.
Successive groupings of one or more image frames may be specified
with the second bit rate being derived from the first frame of the
grouping and maintained constant until the bit rate of the first
frame of a succeeding grouping is detected. The groupings may be
determined on the basis of successive frames within a range of bit
rates as mentioned above or may be specified on other criteria. For
signals encoded according to the MPEG standard, a first grouping
may be specified for I-pictures, with one or more other groupings
for other types of image.
Also in accordance with the present invention there is provided a
video signal encoding apparatus operable to encode a digital video
signal for transmission, the apparatus comprising an encoder stage
operable to encode a received video signal according to a
predetermined coding scheme and to output the signal as a variable
bit-rate data stream and a buffer coupled to receive the said
variable bit-rate data stream from the encoder and arranged to
output a data signal for transmission; characterised by means
operable to detect the bit-rate of the said variable bit-rate data
stream, to derive a second bit rate as a percentage of the encoder
stage output bit rate, which percentage changes in inverse relation
to changes in the encoder stage output rate, and to control the
buffer output data signal bit rate at the said second bit rate.
Further in accordance with the present invention there is provided
an encoded video signal compressed (for transmission) by the method
recited above. The signal, comprising a bit stream of encoded data
for a succession of image frames, has its instantaneous bit rate
inversely related to the bit density of an image frame n frame
periods later where n is determined by the said bit density. As
will be understood, the bit rate of the uncompressed signal is
determined by the bit density, and the n frame periods (where n is
not necessarily an integer) corresponds to the lag introduced by
the tunable delay.
Preferred embodiments of the present invention will now be
described, by way of example only, and with reference to the
accompanying drawings in which:
FIG. 1 schematically represents a series of image frames coded
according to the MPEG standard;
FIG. 2 is a block diagram of part of an encoder apparatus embodying
the invention;
FIG. 3 represents usage of the encoder buffer of the apparatus of
FIG. 2 in relation to decoder buffer usage;
FIG. 4 graphically represents increase in decoder buffer fullness
over time;
FIG. 5 represents a modified version of FIG. 3 embodying the
present invention;
FIG. 6 schematically illustrates the combination of video and MPEG
program streams; and
FIG. 7 represents skew resulting from bit-rate changes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following description is concerned with the management of
MPEG-coded video signals by way of example, although it will be
readily appreciated by the skilled practitioner that the invention
is not restricted to such coding standards.
The MPEG standard describes a syntax and semantics of bit streams
for compressed video and associated audio. Whilst the semantics in
principle specify the functionality of the decoder, the standard
provides no prescriptions for the decoder architecture. Each
decoder will have an input buffer of some sort but how this is to
be realized and what exact size this buffer must have is not
specified.
Recognising the importance of random access for stored video and
the significant bit-rate reduction that may be obtained through
motion-compensated interpolation, the MPEG standard recognises
three types of pictures (frames) namely intra-pictures, predicted
pictures and interpolated pictures, generally referred to as I, P,
and B-pictures respectively. I-pictures provide access points for
random pictures and accordingly have only limited compression.
P-pictures are coded with reference to a past (I or P) picture and
often form a reference for future P-pictures. B-pictures have the
highest degree of compression but require both a past and future
reference for prediction. A typical MPEG sequence of I, P and
B-pictures is shown in FIG. 1.
A schematic representation of an MPEG encoder is shown in FIG. 2. A
received video signal is passed to encoder stage 10 together with a
control signal which varies the quantisation coarseness of the
encoding. In many cases (although not for joint bit rate encoding)
the control signal is kept constant to maintain coarseness at the
same average level. The control signal is derived on the basis of
the incoming video signal by detector 12. The output of the encoder
stage 10 is passed to an encoder buffer 14 at a first bit rate and
also to a calculation unit 16. The calculation unit 16 derives a
value for a second bit rate and passes this to a header insertion
stage 18 which reads the signal bit stream out of the buffer at the
second bit rate and places the bit stream on a communication
channel for transmission to a receiver/decoder. The basis of the
calculations performed by unit 16 to derive an appropriate second
bit rate will now be described in terms of a theoretical
encoder/decoder buffer arrangement.
The MPEG standard constrains bit streams such that they may not
overflow or underflow a hypothetical buffer: this hypothetical
buffer can be related to the physical buffers appearing in the
signal encoder and decoder stages.
In studies of buffer management problems, analysis of the
combination of the encoder buffer and the decoder buffer uses a
model of the two buffers as shown in FIG. 3: this model will be
used to explain the problem that occurs in a variable bit-rate
situation.
The MPEG standard is partitioned in three parts: the video part,
the audio part and the systems part. The video part contains a
mechanism called the VBV and the systems part contains a part
called the transport system target decoder (T-STD). MPEG decoders
are realisable because the signal (bitstream) has to fulfil certain
conditions, and bitstreams that fulfil the VBV condition (or
similar for T-STD) are said to be compliant. Since bitstreams must
fulfil the VBV condition it is possible to design decoders that can
decode compliant bitstreams.
The VBV definition is based on a hypothetical decoder which reads
the compressed pictures in zero time from the VBV buffer and a
channel that continuously fills up the buffer. FIG. 4 shows
characteristic VBV buffer contents as a function of time for a part
of the MPEG frame sequence of FIG. 1. The VBV is specifically
defined for constant bit-rate operation. In case of variable
bit-rate operation, the MPEG standard specifies that the definition
of the VBV buffer is overruled by the definition of the T-STD.
The bitstream is written into the buffer at the bit-rate specified
in the sequence header. The bitstream contains information about
the contents of the VBV buffer. This information is presented in a
field in each picture header called the VBV delay, which specifies
the time between the moment that the header enters the VBV buffer
and the moment of decoding the picture.
In the systems part of the MPEG standard, mechanisms are defined
that provide means to synchronise the decoder. These mechanisms are
called the decoding time stamp (DTS) and the presentation time
stamp (PTS) which are measured in time units. The mechanism of
system clock reference (SCR) is used to recover the correct time.
The complete decoding operation is described again in terms of a
hypothetical decoder which is referred to as the standard target
decoder (STD). The systems part describes two types of systems
layer: the transport stream and the program stream. The STD for the
transport stream is referred to as T-STD and for the program stream
the term P-STD is used. In the following the difference between
these streams is explained.
In principle the program stream is defined for applications which
have a very low error probability such as optical recording. The
basic characteristic of the program stream is that it can convey
only one program and that the bit-rate can be varying: as such it
may be of limited general use. Of greater utility is the transport
stream, which is intended for multi-program environments. The
bit-rate for each program is allowed to vary but the sum of all
bit-rates must be constant. The transport stream uses a fixed size
packets of 188 bytes. The T-STD can be used for HD-CD (VBR optical
medium) by reading a variable bit-rate from the disk and adding an
artificial empty program to the bitstream in passing the
information from the reader on to the decoder such that the total
bit-rate is constant. The definition of the functionality of both
the P-STD and the T-STD is similar.
We have recognised that a problem in conventional buffer-management
is that the available buffer space in the decoder depends on the
bit rate. At low bit rate we have too small an effective decoder
buffer capacity available, which will hamper the image quality.
Referring back now to the model of FIG. 3, the buffering model uses
the following parameters
the (discrete) time n
the current bit rate R[n] of readout from the encoder buffer
the input-output delay d
physical encoder buffer size B.sub.E
physical decoder buffer size B.sub.D
encoder buffer pointer E[n]
decoder buffer pointer D[n]
range of decoder buffer pointer D.sub.min
[n].ltoreq.D[n]<D.sub.max [n]
range of encoder buffer pointer E.sub.min
[n].ltoreq.E[n]<E.sub.max [n]
It will be noted that the range of the buffer pointers is
introduced as a separate variable which is time dependent, with the
current buffer pointers E[n] and D[n] representing the buffer
contents. A key variable in the buffer system is the input-output
delay d which denotes the time between inputting a picture to the
encoder and the time that this picture is decoded by the decoder.
In the following derivations, we assume d to be an integer number
of pictures, although non-integer values are possible as well. This
variable is necessarily constant over the entire sequence since
otherwise a non-continuous display would result, affecting motion
portrayal.
We assume that the buffers are operated in the following way
In this model we assume an action (next n) every new picture. From
this buffer behaviour model the following relation between the
encoder buffer contents and the decoder buffer contents can be
derived ##EQU1##
From Equation 2 we can derive relation between the boundaries of
the buffer contents which yields ##EQU2##
This equation shows that underflow in the encoder is linked to
overflow in the decoder and vice versa; the encoder buffer and
decoder buffer have dual behaviour. Clearly, there is a lag of d
samples between encoder and decoder constraints.
A common choice (that is made in the Reibman and Haskell paper
mentioned previously) is to select:
This choice means that underflow in either of the two buffers is in
fact a physical underflow and, hence, a loss of data. Underflow in
the encoder means the following violation on the decoder side
of
with ##EQU3##
Hence the maximum available buffer space in the decoder depends on
the (integrated) bit rate over the past d samples. This may lead to
problems if, for instance, the bit rate is constant over more than
d samples. In such a situation we have
hence at a low bit-rate we have a small effective decoder buffer
available. At a low bit-rate we still may have relatively large
I-pictures, which can happen for scenes with high detail and
absolutely no motion. Since the available buffer space is small, it
is a conventional technique to reduce the quality of that I-picture
such that the buffer constraint in Equation 5 is not violated.
Our solution to this problem is to have a small delay, as shown in
FIG. 5 and also to use a relatively high bit-rate for the
I-picture. The small delay averages out over a small number of
pictures, and hence a small bit-rate peak will help. The special
treatment of I-pictures means that over a series of pictures such
as the sequence of FIG. 1, we use two bit-rates; a relatively high
one for the period of time containing the I-pictures and a lower
one for the other classes of pictures. This provides us with higher
peak bit-rates. For the sequence of FIG. 1, we therefore have three
groups of pictures (GOP's), one for each of the I-pictures and one
for the GOP of P- and B-pictures between them, with the bit-rate
remaining constant (either high or low) in each GOP. If desired, a
third bit-rate (between the relatively high and relatively low
levels) may be specified for P-pictures in which case the sequence
of FIG. 1 would comprise five GOP with the bit-rate following the
pattern of high-low-medium-low-high over the series.
The use of this specified bit-rate technique in the encoder may be
detected from the decoder by testing Equation 5 on the bit stream.
If the equation yields an "overflow" it shows the technique to have
been used.
The choices for our larger encoder buffer and appropriate bit-rate
over time will now be described. The conventional choice made in
Equation 4 is the source for existing problems: we have appreciated
that by making an alternative choice more flexibility in the
decoder is provided. We choose
which may be considered as an advance selection of the largest
possible decoder buffer. As will be shown, this choice leads to a
larger encoder buffer. Note that an alternative choice for the
buffer, such as ##EQU4##
will yield similar conclusions for the encoder buffer, unless A=0,
B=1 (which is the case in the original problem).
Substituting the choices from Equation 8 in Equation 3 yields
##EQU5##
These equations show that the lower limit of the encoder buffer
contents is non-zero which means that the encoder buffer must be
larger than is conventional. In fact the physical size of the
encoder buffer equals that of the decoder buffer plus some margin.
The margin is calculated from the maximum and minimum bit rate on
the channel as will be demonstrated below.
Considering Equation 10 for our analysis, analysis of the extremal
values of E.sub.min yields insight in important system
characteristics. One end of the scale is reached when E.sub.min =0.
This situation will occur if we transmit for more than d symbols at
the minimum bit rate. Substituting these assumptions in Equation 10
yields
Thus we use the minimum bit rate to select the required delay. It
is appreciated that this delay will generally be larger than in the
conventional situation, but the actual value remains acceptable,
especially in the case of recording applications.
At the other end of the scale we find the required extra buffer
size by considering that the encoder buffer size can be written
as
where the margin is equal to the maximum value of E.sub.min. The
maximum value is reached if the bit rate is at its maximum value
for more than d samples (pictures). In this case we find
##EQU6##
Turning now to the calculation of the time dependent bit rate R[n],
the decoder buffer content is required to be the same at the
beginning and at the end of a GOP. FIG. 4 illustrates this
condition using the decoder buffer fullness.
From Equation 1 it can be seen that this condition means
##EQU7##
and keeping the bit rate R[i]=R constant over a GOP of length N
pictures we have ##EQU8##
From Equation 15 it will be appreciated that the channel bit rate
has to change d samples after the source bit rate has changed and
also that the bit rate will generally have to be changed within a
picture, rather than at the header of a picture, although this may
occasionally occur. The bit rate changes are shown in terms of the
MPEG syntax in FIG. 6.
The MPEG transport stream is meant for multi-program environments
and the program stream is meant for environments with one program,
like optical recording. FIG. 6 shows how a video bit stream can be
wrapped in a program stream. In the program stream we have the MPEG
concepts PACK-headers and PES-headers (PES-packetized elementary
stream). The PACK-header contains information on the elementary
stream rate (=ES-rate). Since PACK-headers are not necessarily
constrained to precede picture-headers or PES-headers we are
allowed to change the bit rate at any moment.
The consequences of Equation 15 and Equation 16 are shown in FIG. 7
which shows the skew between moment of inputting the groups of
pictures to the buffer and the moment of changing the bit rate: the
continuous lines represent the time interval in which the bits
resulting from compressing a GOP are written into the buffer, and
the dashed lines indicate the time interval for which the output
bit rate calculated for that GOP is applied.
In the foregoing, we have shown an enhanced buffer management
strategy that does not put constraints on the quality of
compression, for only a small extra delay.
From reading the present disclosure, other modifications will be
apparent to persons skilled in the art. Such modifications may
involve other features which are already known in the design,
manufacture and use of digital video signal encoding and decoding
systems and devices and component parts thereof and which may be
used instead of or in addition to features already described
herein.
* * * * *